1. Field of the Invention
The present invention relates to a nanostructure and a method of producing the same. The nanostructure produced by anodizing aluminum according to the present invention may be used in a wide variety of applications such as functional materials for use in electronic devices or micro devices. Specific examples include quantum effect devices, electrochemical sensors, biosensors, magnetic memories, magnetic devices, light emitting devices, photonic devices, solar cells, etc.
2. Description of the Related Art
In thin films, fine wires, and fine dots of metal or semiconductor, if motion of electrons is restricted within a region smaller than a particular length, the thin films, the fine wires, or the fine dots often exhibit special electric, optical, and/or chemical characteristics. From this point of view, materials having a fine structure (nanostructure) with a size smaller than 100 nm are attracting increasing attention as functional materials.
One known method of producing nanostructures is to employ a semiconductor processing technique including a fine pattern writing technique such as photolithography, electron-beam lithography, and X-ray lithography.
In addition to the production method described above, a self-forming technique is being developed. In this technique, a self-formed periodic structure is used to realize a novel nanostructure. This technique has a potential ability to produce a peculiar nanostructure including a finer structure, depending on a fine structure used as a base, than can be obtained by the conventional technique, and thus a lot of investigations are being performed.
An example of a self-formed peculiar structure is an anodized aluminum oxide film (refer to, for example, R. C. Furneaux, W. R. Rigby and A. P. Davidson, NATURE, Vol. 337, P. 147 (1989)). If an aluminum plate is anodized in an acid electrolyte, a porous oxide film is formed. FIG. 3A is a cross-sectional view schematically illustrating a nanostructure obtained by anodizing an aluminum plate 31 so as to form a porous anodized film 32 on the surface of the aluminum plate 31. FIG. 3B is a cross-sectional view schematically illustrating a nanostructure obtained by anodizing the surface of a thin aluminum film 34 formed for example on a semiconductor substrate 33 so as to form a porous anodized film 32. As can be seen from FIGS. 3A and 3B, the feature of the anodized film is that it has a peculiar geometric structure including very small cylindrical holes (nanoholes) 35 which have diameters 2r ranging from several nm to several hundred nm and which are arranged in parallel at intervals of several ten nm to several hundred nm. The cylindrical nanoholes 35 have a large aspect ratio and have good uniformity in terms of the diameter over the entire length.
The diameter 2r of the nanoholes 35 and the hole-to-hole distance 2R can be controlled to a certain extent by adjusting the current and voltage during the anodization process. There is a barrier layer (aluminum oxide layer) 36 between the anodized film 32 and the aluminum substrate 31 or the aluminum film 34. A various applications are being attempted to take the advantages of such peculiar geometric structures obtained in anodized films. For example, anodized films may be used as films having high abrasion resistance and high dielectric strength. An anodized film may be separated from an underlying material and may be used as a filter. Furthermore, by filling the nanoholes with metal or semiconductor or by using a replica of nanoholes, other various application are also possible, such as coloring, magnetic storage media, EL devices, electrochromic devices, optical devices, solar cells, and gas sensors. The anodized film is also expected to have further various applications such as quantum effect devices (quantum fine wires, MIM (metal-insulator-metal) devices), molecular sensors using nanoholes as chemical reaction spaces, etc. (Masuda, Solid State Physics, 31, 493 (1996))
Producing nanostructures using semiconductor processing techniques is problematic because of low production yield and high apparatus cost. A simpler technique of producing nanostructures with good reproducibility is therefore desirable. From this point of view, the above-described self-forming techniques, in particular the technique of anodizing aluminum, have the advantage that nanostructures can be easily produced with high controllability. These techniques are also useful to produce large-area nanostructures.
The nanostructures shown in FIG. 3A and 3B have limitations in terms of shapes and applications because nanostructures can be formed only on the surface of an aluminum plate (film). For example, because the melting point of aluminum is as low as 660.degree. C., the nanoholes formed on the surface of aluminum cannot be subjected to a heat treatment at temperatures higher than 660.degree. C. Therefore, to use nanoholes as functional materials in various applications, it is necessary to develop a technique of forming an anodized film on a substrate with a high melting point without destroying its peculiar geometric structure, and also a technique of preventing generation of cracks at high temperatures.
On the other hand, to use the peculiar geometric structure of the anodized film in an electron device, an anodized film must be formed on a semiconductor substrate. In particular, a technique of forming an anodized film on a silicon substrate is important. If it is possible to form an anodized film on a silicon substrate, then it becomes possible to integrate a nanostructure with a silicon semiconductor device such as a diode and a transistor. This allows the nanostructure to be used in wider applications.
A technique of forming an anodized film including nanoholes on a silicon substrate is disclosed in Japanese Patent Laid-Open No. 7-272651. In this technique, an aluminum film is first formed on a silicon substrate, and then the aluminum film is converted into an anodized film. After that, the barrier layer of the anodized film, present at the bottom of the nanoholes, is removed. A metal layer (Au, Pt, Pd, Ni, Ag, Cu) capable of forming an eutectic alloy with silicon is then formed on the exposed parts of the silicon substrate and silicon capillary crystal is grown using the VLS method. In this technique, to produce nanoholes which are completely cut through an anodized film from its surface to a silicon substrate, the barrier layer at the bottom of the nanoholes is removed after anodizing the aluminum film. The removal of the barrier layer may be performed, for example, by means of etching using a chromic acid-based etchant or by means of keeping a silicon substrate, together with an opposite electrode electrically connected to the silicon substrate via an external wire, in a solution still after completion of anodization.
The inventors of the present invention have investigated the above-described technique disclosed in Japanese Patent Laid-Open No. 7-272651. The investigation has revealed that it is very difficult to completely anodize an aluminum film over its entire thickness such that a barrier layer remains at the bottom of all nanoholes 35. That is, the depth of nanoholes varies more or less, and thus it is difficult to produce a structure having a remaining barrier layer with an uniform thickness over a wide area as shown in FIG. 4. During the process of anodizing the aluminum film, the barrier layer is altered or lost in a very short time although the reason is not clear. As a result, the electrolyte comes into contact with the silicon substrate. Thus, oxidation of the silicon substrate and decomposition of the electrolyte occur. Although nanoholes having a remaining barrier layer can be formed in a certain area on the substrate, if the barrier layer is removed, then, as shown in FIG. 5, the diameter of the nanoholes 37 in the parts where the barrier layer is removed will not be uniform in the resulting structure. Furthermore, the shape varies greatly from one nanohole to another. In particular when nanoholes have a large depth, the anodized film tends to have a nonuniform thickness and anodization tends to occur nonuniformly. Thus, it is very difficult to form completely-cut-through nanoholes having an uniform shape with good repeatability.